The present invention relates to a pulse-radar system and more particularly to a moving target indicator which cancels the unwanted component signal (clutter) in a received signal and detects a moving target signal.
Conventionally, a pulse-radar system indicating a moving target such as an airplane is designed with a clutter-canceller or suppressor to remove stationary (ground) clutter from buildings or other surface irregularities and moving clutter, such as weather clutter from moving reflectors, such as rain or clouds and to detect and indicate the moving target. Among these clutter removing means, a moving target indicator system, hereinafter simply referred to as an MTI, is often used for removing stationary clutter.
Generally, an MTI system adapted as a radar system for detecting and indicating aircraft is used to remove or cancel a signal from a stationary clutter so as to indicate only a moving target. When the phases of transmitted R.F. pulses are compared with the phases of received R.F. pulses, the signal from a stationary clutter always has a constant phase, whereas the signal from a moving target always has a different phase for each pulse repetition period. Hence, phase-detection is carried out in response to signals from the same distance for the continued 2 pulse-repetition periods and the difference between the obtained video signals is taken so as to cancel out the video signal from a stationary clutter and to leave only the video signal corresponding with a moving target. This kind of MTI radar system is described in chapter 17, "RADAR HANDBOOK", edited by Merill Skolnik, McGraw-Hill, U.S.A., 1970.
However, an ordinary MTI system on a moving carrier, such as a ship, cannot cancel out moving clutter from rain, fog, the sea surface, moving ground clutter from land, or the like. This is, because these signals have a different phase for each pulse repetition period, similar to said signals from a moving target.
An adaptive MTI system is suited for cancelling moving clutter, including relatively moving ground clutter as stated before, and for indicating only a moving target such as an airplane. Cancelling an echo from a moving object can be accomplished by varying and transferring Doppler frequency of moving clutter to a notch of an MTI filter. For this kind of conventional adaptive MTI radar system, a clutter-locking MTI system is utilized. The details thereof are described in the chapter 9, "RADAR DESIGN PRINCIPLES" McGraw-Hill, U.S.A., 1969.
A clutter-locking MTI system can remove moving clutter or stationary clutter by detecting an average Doppler frequency (or an average Doppler phase-shift) and locking the average Doppler frequency (or the average Doppler phase-shift) to a notch of the MTI filter. However, when stationary clutter and moving clutter are present, it is not possible to sufficiently remove both because the detected average Doppler frequency, that is, average Doppler phase-shift, is different from that of stationary clutter alone or of moving clutter alone. Further details will be explained in this respect. When both stationary clutter and moving clutter are present, the input signals of the clutter-locking MTI canceller are; ##EQU1## where V.sub.1 and V.sub.2 denote an input signal (radar signal) at the received time point and that after a radar-received repetition time T, respectively. The first and second terms of the equation (1) denote stationary clutter components including the Doppler frequency fd.sub.1 (in this case, fd.sub.1 =0) and moving clutter components including the Doppler frequency fd.sub.2, respectively. E.sub.1 and E.sub.2 denote amplitude components of the stationary clutter and the moving one and .phi..sub.0 denotes a phase defined on the basis of the existing location of clutter. Generally, E.sub.1 .noteq.E.sub.2 and fd.sub.1 .noteq.fd.sub.2.
When one input signal is phase-shifted by the presumptive value .phi.d=2fdT of the average Doppler phase-shift, the canceler output is: EQU .DELTA.V=2E.sub.1 sin [.pi.(fd.sub.1 -fd)T] cos [2.pi.fd.sub.1 t+.phi..sub.0 +.pi.(fd.sub.1 -fd)T]+2E.sub.2 sin [.pi.(fd.sub.2 -fd)T] cos [2.pi.fd.sub.2 t+.phi..sub.0 +.pi.(fd.sub.2 -fd)T]
where there exists the condition of fd=fd.sub.1 .noteq.fd.sub.2 or fd=fd.sub.2 .noteq.fd.sub.1. Thus, it is impossible to remove both stationary clutter and moving clutter.